This invention relates to energy-efficient, laser-based methods and systems for processing target material. In particular, this invention relates to the use of a pulsed laser beam to ablate or otherwise alter a portion of a circuit element on a semiconductor substrate, and is particularly applicable to vaporizing metal, polysilicide and polysilicon links for memory repair. Further application can be found in laser-based micromachining and other repair operations, particularly when it is desired to ablate or modify a microscopic structure without damaging surrounding areas and structures, which often have non-homogeneous optical and thermal properties. Similarly, the material processing operations can be applied to other microscopic semiconductor devices, for instance microelectromechanical machines. Medical applications may also exist, such as microscopic tissue or cell ablation with miniature fiber optic probes.
Semiconductor devices such as memories typically have conductive links adhered to a transparent insulator layer such as silicon oxide, which is supported by the main silicon substrate. During laser processing of such semiconductor devices, while the beam is incident on the link or circuit element, some of the energy also reaches the substrate and other structures. Depending upon the power of the beam, length of time of application of the beam, and other operating parameters, the silicon substrate and/or adjacent can be overheated and damaged.
Several prior art references teach the importance of wavelength selection as a critical parameter for substrate damage control. U.S. Pat. Nos. 4,399,345, 5,265,114, 5,473,624, 5,569,398 disclose the benefits of wavelength selection in the range beyond 1.2 um to avoid damaging silicon substrates.
The disclosure of the above-noted ""759 patent further elaborates on the wavelength characteristics of silicon. The absorption in silicon rapidly drops off after about one micron with an absorption edge of about 1.12 microns at room temperature. At wavelengths greater than 1.12 microns, the silicon starts to transmit more and more easily and, thus, it is possible to obtain better part yields upon removing material from the silicon. In the range around 1 micron the absorption coefficient decrease by a factor of four orders of magnitude going from 0.9 microns to 1.2 microns. In going from the standard laser wavelength of 1.047 microns to 1.2 microns the curve shows a drop of two orders of magnitude. This shows a drastic change in absorption for a very slight change in wavelength. Thus, operating the laser at a wavelength beyond the absorption edge of the substrate circumvents damage to the substrate, which is especially important if there is a slight misalignment of the laser beam with respect to the link or where the focused spot extends beyond the link structure. Furthermore, if the substrate temperature rises during processing the absorption curve shifts will shift further into the infrared which can lead to thermal runaway conditions and catastrophic damage.
The problem of liquid crystal repair is similar to the problem of metal link ablation. The wavelength selection principle for maximizing absorption contrast was advantageously applied in the green wavelength region in a manner analogous to the above disclosures for the same purposexe2x80x94namely removal of metal without substrate damage. The system manufactured by Florod is described in the publication xe2x80x9cXenon Laser Repairs Liquid Crystal Displaysxe2x80x9d, LASERS AND OPTRONICS, pages 39-41, April 1988.
Just as wavelength selection has proven to be advantageous, it has been recognized that other parameters can be adjusted to improve the laser processing window. For example, it was noted in xe2x80x9cComputer Simulation of Target Link Explosion in Laser Programmable Redundancy for Silicon Memoryxe2x80x9d by L. M. Scarfone and J. D. Chlipala, 1986, p. 371, xe2x80x9cIt is desirable that laser wavelengths and various material thicknesses be selected to enhance the absorption for the link removal process and reduce it elsewhere to prevent damage to the remainder of the structure.xe2x80x9d The usefulness, in general, of thicker insulative layers underneath links or circuit elements and the usefulness of limiting the duration of heating pulses has also been recognized, as in the paper co-authored by the applicant, xe2x80x9cLaser Adjustment of Linear Monolithic Circuits,xe2x80x9d Litwin and Smart, 100/L.I.A., Vol. 38, ICAELO (1983).
The ""759 patent teaches the tradeoffs that exist with selection of the longer wavelengthsxe2x80x94specifically compromises with respect to spot size, depth of focus, and pulse width, available from Nd:YAG lasers. These parameters are of critical importance for laser processing at increasingly fine dimensions, and where the chances of collateral damage to surrounding structures exist.
In fact, any improvement which widens the processing window is advantageous as the industry continues to push toward higher density microstructures and the associated geometries which are a fraction of one micron in depth or lateral dimension. The tolerances of energy control and target absorption become large compared to the energy required to process the microstructure at this scale. It should be noted from the above discussion that laser processing parameters are not necessarily independent in micromachining applications where a small laser spot, about 1 xcexcm, is required. For instance, the spot size and pulse width are generally minimized with short wavelengths, say less than 1.2 xcexcm, but the absorption contrast is not maximized. Makers of semiconductor devices typically continue production of earlier developed products while developing and entering production of more advanced versions that typically employ different structures and processes. Many current memory products employ polysilicide or polysilicon links while smaller link structures of metal are used for more advanced products such as the 256-megabit memories. Links of 1 micron width, and ⅓ micron depth, lying upon a thin silicon oxide layer of 0.3 to 0.5 microns are being used in such large memories. Production facilities traditionally have utilized Q-switched diode pumped YAG lasers at and related equipment capable of operating at the conventional wavelengths of 1.047 xcexcm-1.32 xcexcm and related equipment capable of operating in the wavelength region recognized for its lower absorption by silicon. However, these users also recognize the benefits of equipment improvements which results in clean severing of link structures without the risk of later chip failures due to conductive residue or contamination near the ablation site.
Other degrees of freedom include laser pulse energy density (delivered to the target) and pulse duration. It has been taught in the prior art that pulse width should be limited to avoid damage in micromachining applications. For example, in U.S. Pat. No. 5,059,764 a laser processing workstation is disclosed wherein a q-switched laser system is utilized to produce, among other things, relatively short pulses on the order of 10-50 ns. It was disclosed that for material processing applications (like semiconductor memory repair via link blowing and precision engraving), the output pulse width should be relatively shortxe2x80x94and that a pulse width less than 50 ns is required in many applications, for example 30 ns. The proper choice of pulse width allows for ablation (evaporation without melting).
High speed pulsed laser designs may utilize Q-switched, gain switched, or mode-locked operation. The pulse duration and shape of standard Q-switched and other pulsed lasers can be approximated at a fundamental level by integrating the coupled rate equations describing the population inversion and the photon number density relative to the lasing threshold at the start of the pulse. For the Q-switched case, on a normalized scale, a higher number of atoms in the inverted population relative to the threshold the faster the pulse rise time, the narrower the width, and the higher the peak energy. As the ratio decreases the pulse shape becomes broader with lower energy concentration.
Often Q-switched laser pulses resemble a Gaussian temporal distribution, or a mixture of a Gaussian with an exponential decaying tail. As disclosed in the ""759 patent, the shorter wavelength diode pumped systems are capable of producing relatively short pulses, about 10 ns, when measured at the half power points (i.e., standard definition of pulse duration) and are operated in a favorable wavelength region. Despite successful operation, applicant has found several limitations associated with the temporal pulse shape characteristic of standard diode pumped Q-switch laser systems, including the practical rise time limitations, the power distribution between the half maximum points, and the pulse decay characteristic which, when improved using the method and system of the present invention, provided noticeably better results in a metal link blowing application.
Throughout the remainder of this specification, xe2x80x9cpulse shapingxe2x80x9d refers to the generation of a laser pulse which is to be detected with a detector of electromagnetic radiation where xe2x80x9cshapexe2x80x9d refers to the power on the detector as a function of time. Furthermore, xe2x80x9cpulse widthxe2x80x9d or xe2x80x9cpulse durationxe2x80x9d refers to the full width at half maximum (FWHM) unless otherwise stated. Also, Q-switched pulses collectively refers to temporal distribution of pulses obtained, for example, in standard Q-switched systems which may resemble a mixture of a substantially Gaussian central lobe with a relatively slow decaying exponential tail. These wave shapes are formally referred to as a xe2x80x9cQ-switched pulse envelopexe2x80x9d in laser literature. FIG. 1c shows such pulses.
In U.S. Pat. No. 5,208,437 (i.e., the ""437 patent), a pulse width specification of less than 1 ns was specified for a memory repair application. Earlier work by the co-inventors of the ""437 patent disclosed in xe2x80x9cLaser Cutting of Aluminum Thin Film With No Damage to Under Layersxe2x80x9d, ANNALS OF THE CIRP, Vol 28/1, 1979, included experimental results with relatively short laser pulses having a xe2x80x9cGaussianxe2x80x9d shape as defined above. The results indicated a xe2x80x9cdesired portion of the interconnection patternxe2x80x9d which is made of aluminum or the like, xe2x80x9ccan be cut without the layer disposed below the interconnection pattern being damagedxe2x80x9d. Specifications for the pulse width of substantially 1 ns or less with energy density of substantially 106W/cm2 were disclosed for the apparatus. However, there was no disclosure regarding a method of temporal pulse shaping, although spatially the beam was shaped to correspond to the interconnection pattern. Furthermore, applicant""s analysis on high density memory devices having multiple layers with specified pulsewidths in the ultrafast range, which is approached with the 100-300 ps used in the ""437 patent, have not been satisfactory. Overcoming this limitation would presently require the ultrafast laser system to produce multiple pulses for processing each target site which would slow the laser processing rate to an unacceptable level.
Continuing to the ultrafast scale, experimental results have been disclosed for micromachining operations. The ultrafast pulses have durations on the order of fs (10-15 sec) to ps (10-12) and, at the decreased scale, exploit material properties at the atomic and molecular which are fundamentally different than found in the range of several hundred ps to ns.
In U.S. Pat. No. 5,656,186 and the publication xe2x80x9cUltrashort Laser Pulses tackle precision Machiningxe2x80x9d, LASER FOCUS WORLD, August 1997, pages 101-118, machining operations at several wavelengths were analyzed, and machined feature sizes significantly smaller than the diffraction limited spot size of the focused beam were demonstrated.
Laser systems for ultrafast pulse generation vary in complexity and are exemplary embodiments are described in U.S. Pat. Nos. 5,920,668 and 5,400,350, and in Ultrafast Lasers Escape The Labxe2x80x9d, PHOTONICS SPECTRA, July 1998, pp. 157-161. The embodiments generally include methods for pulse stretching of mode locked ultrafast pulses prior to amplification to avoid amplifier saturation followed by compression to extremely narrow widths. This technology holds promise for certain class of micromachining and possibly finer scale xe2x80x9cnanomachiningxe2x80x9d operations, the latter benefit afforded by machining below diffraction limit. However, Applicant has discovered practical limitations at the present time with the available power in each pulse for applications like metal link blowing and similar micromachining applications leading to the unacceptable requirement for multiple pulses.
Applicant wishes to elaborate on the rationale for the use of a short pulse, fast rise time pulse is indicated in the following paragraphs as the reasons are manifold and a number of theoretical and empirical papers and books have been written on the subject. Ablation of metal links is taken as an example, although the principles extend to many laser processing applications where a target material is surrounded by material having substantially different optical and thermal properties. The following references 1-3 are examples:
1. John F. Ready, Effects of High Power Laser Radiation, ACADEMIC PRESS, New York 1971, pages 115-116.
2. Sidney S. Charschan, Guide for Material Processing By Lasers, Laser Institute of America, The Paul M. Harrod Company, Baltimore Md., 1977, pages 5-13.
3. Joseph Bernstein, J. H. Lee, Gang Yang, Tariq A. Dahmas, Analysis of Laser Metal-Cut Energy Process Window (to be published).
Metal Reflectivity
Metal reflectivity decreases with increased power density of a laser pulse (ref. 1). The reflectivity of a metal is directly proportional to the free electron conductivity in a material. At high electric field densities as delivered by a high intensity laser, the collision time between electrons and the lattice is reduced. This shortening of the collision time reduces the conductivity and hence the reflectivity. For example, the reflectivity of aluminum decreases from 92% to less than 25% as the laser power densities increases to the range of 109 watts/cm2. Hence, to circumvent the loss of laser energy to reflection it is advantageous to achieve high power density at the work piece in as short a time as possible.
Thermal Diffusivity
The distance D that heat travels during a laser pulse is proportional to the laser pulse width as follows:   D  =      kt  
where:
K is the thermal diffusivity of the material; and
t is the length of the laser pulse.
Hence, it can be seen that a short laser pulse prevents heat dissipating to the substrate below the melting link and also heat conducting laterally to the material contiguous to the link. However the pulse must be long enough to heat the link material all the way through.
Thermal Stress and Link Removal
Through the absorption of the laser energy the target metal link heats up and tries to expand. However, the oxide surrounding the link contains the expanding material. Hence, stress is built up within the oxide. At some point the pressure of the expanding metal exceeds the yield point of the oxide and the oxide cracks and the metal link explodes into a fine particle vapor. The principal crack points of metal link occurs at the maximum stress points, which are at the edges of the link both top and bottom as shown in FIG. 1b. 
If the oxide over the link is somewhat thin then the cracking of the oxide will occur at the top of the link only and the oxide and link will be removed cleanly as shown in FIG. 1a. However, if the oxide is somewhat thick, cracking can occur at the bottom of the link as well as the top and the crack will propagate down to the substrate as shown in FIG. 1b. This is a highly undesirable circumstance.
Q-switched laser systems can be modified to provide short pulses of various shapes. Typical prior art lasers that produce high peak power, short pulse lasers are standard Q-switched lasers. These lasers produce a temporal pulse having a moderate pulse rise time. It is possible to change this temporal shape by using a Pockels Cell pulse slicer that switch out sections of the laser beam. In U.S. Pat. No. 4,483,005 (i.e., the ""005 patent), invented by the Applicant of the present invention and having the same assignee, various methods for affecting (i.e., reducing) laser beam pulse width are disclosed. As taught in the ""005 patent, which is hereby incorporated by reference, the laser pulse can be shaped somewhat to produce a xe2x80x9cnon-Gaussianxe2x80x9d shaped beam by truncating energy outside the central lobe. It should be noted that if a relatively broad Q-switched waveform is to be transformed to a narrow, uniform shape, only a small fraction of the pulse energy will be used. For example, truncation of a Gaussian pulse to provide a sharp rise time and a narrow pulse with flatness to within 10% reduces the pulse energy by about 65%.
Similarly, in U.S. Pat. No. 4,114,018 (the ""018 patent), temporal pulse shaping to produce square pulses is disclosed. FIG. 7 shows the time interval for relatively flat laser power output. In the ""018 patented method, it is necessary to remove a temporal segment of the beam intensity in order to generate the desired pulses.
A desirable improvement over the prior art would provide an efficient method for generating short pulses with high energy enclosure within the pulse duration with rapidly decaying tails. In order to accomplish this, laser technology which produces pulse shapes different than those of the Q-switched pulse envelope is preferred. Such pulses have fast rise time, uniform energy in the central lobe, and fast decay.
The fast rise-time, high power density pulse as produced by a laser other than a standard Q-switched Nd:YAG will best accomplish this task.
These benefits are implemented in a preferred manner in a system which uses laser technology departing significantly from the traditional Q-switched, solid state diode or lamp pumped, YAG technology.
Improvements over the prior art are desired with a method and system for generating pulses having a shape which is different than standard Q-switched pulsesxe2x80x94pulses having faster rise time, relatively uniform and higher energy concentration in the central lobe, and fast fall time.
Applicant has determined that improved results can be obtained in applications of metal link blowing. For instance, a non-Gaussian, substantially rectangular pulse shape is particularly advantageous for metal link processing where an overlying insulator exists. Applicants results show that the fast rise time on the order of 1 ns, and preferably about 0.5 ns, provides a thermal shock to the overlying layer of oxide which facilitates the link blowing process. In addition, at the higher power density the reflectivity is reduced with the fast rising short pulse. A pulse duration of about 5 ns with a substantially uniform pulse shape allows more energy to be coupled to the link leading to a reduced energy requirement for link removal. Rapid fall time of about 2 ns is important to eliminate the possibility of substrate damage. Furthermore, an advantage of a nearly square power density pulse in time is that the power density is the highest when it is needed and the pulse is off when it is not.
A short fast rising pulse will allow the top of the link to melt and expand first before the heat can diffuse down to the lower portion of the link. Hence, stress is built up in the top of the link and promotes cracking of the top layer without generating a crack down to the substrate.
It is an object of this invention to provide a compact, gain switched laser system which has the capability for generating sub-nanosecond rise time pulses having short duration of a few nanoseconds and rapid fall time. State of the art fast pulse systems incorporate gain switched technology, in which a low power semiconductor seed laser is rapidly and directly modulated to produce a controlled pulse shape which is subsequently amplified with a laser amplifier, such as a cladding pumped fiber optic system with a high power laser diode or diode array used as the pump laser. Such laser systems are described in U.S. Pat. No. 5,694,408 and PCT Application No. PCT/U.S98/42050, and are xe2x80x9cbuilding blocksxe2x80x9d of certain ultra-fast chirped pulse amplifier systems, for instance the system described in U.S. Pat. No. 5,400,350.
It is a general object of the invention to improve upon prior art laser processing methods and systems, particularly those where the optical and/or thermal properties of a region near the target material differ substantially.
It is a general object of the invention to provide laser pulse shaping capability for micromachining and laser material processing applications, for instance laser ablation of links or other interconnects on semiconductor memories, trimming, drilling, marking, and micromachining. A predetermined waveform shape is generated from a gain-switched laser which is different than that of the standard Q-switched systems.
It is an object of the invention to provide improvements and margin for semiconductor processing, for example, 16-256 megabit semiconductor repair, which results in clean processing of microstructures without the risk of later device failure due to conductive residue or contamination near the ablation site.
It is an object of the invention to provide a pulse waveform rise time in as short as a few hundred picoseconds, the pulse duration typically less than about 10 nanoseconds with rapid pulse decay, thereby providing laser processing of a target structure at high power density, whereby damage arising from thermal shock and diffusion in the surrounding regions is minimized.
It is an object of the invention to prevent damage to the structures surrounding and beneath the target material in semiconductor laser processing applications by achieving high power density at the workpiece in a very short time with a high power, fast rise time pulse at any wavelength suitable for the laser ablation process thereby improving the process window in a semiconductor material processing application.
It is an object of the invention to process a target site with a single laser processing pulse with rise time fast enough and with sufficient power density so as to provide a reduction in the reflectivity of a metal target structure, such a single metal link on a semiconductor memory, and hence provide more efficient coupling of the laser energy. The fast rising laser pulse is of sufficient pulse duration to efficiently heat and vaporize the material of each metallic target structure with relatively uniform power density during the ablation period, yet a rapid pulse fall time after the target material is vaporized avoids damage to surrounding and underlying structures.
It is an object of the invention to provide superior performance in semiconductor metal link blowing applications when compared to systems utilizing standard Q-switched lasers, such lasers having typical pulse rise times of several nanoseconds and represented by a Q-switched pulse envelope. A laser pulse is generated to provide a substantially square pulse shape with pulse duration in the range of about 2-10 nanoseconds and a rise time of about 1 ns and preferably about 0.4 ns. Additionally, the pulse decay is to be rapid when switched off thereby allowing only a very small fraction of pulse energy to remain after the predetermined pulse duration, the pulse xe2x80x9ctailsxe2x80x9d rapidly decaying to a sufficiently low level so as to avoid the possibility of damaging the underlying substrate or other non-target materials. A comparison of these pulses is illustrated in FIG. 2.
It is an object of the invention to expand the processing window of a semiconductor laser ablation process to provide rapid and efficient ablation of microscopic structures surrounded by materials having different optical and thermal properties. Such structures are typically arranged in a manner where the width and spacing between the structures is about 1 micron or smaller and stacked in depth. The application of a short laser pulse cleanly ablates the target material, yet damage to surrounding materials caused by heat dissipation in either the lateral direction or damage to the underlying substrate below the target material is prevented.
It is an object the invention to controllably machine a material having substantially homogeneous optical and thermal properties with the application of a short pulse having high energy density, the pulse duration being a few nanoseconds in the material processing range where a fluence threshold is approximately proportional to the square root of laser pulse width.
In carrying out the above objects and other objects of the present invention, an energy-efficient, laser-based method for processing target material having a specified dimension in a microscopic region without causing undesirable changes in electrical or physical characteristics of material surrounding the target material is provided. The method includes generating a laser pulse train utilizing a laser having a wavelength at a repetition rate wherein each of the pulses of the pulse train has a predetermined shape. The method then includes optically amplifying the pulse train without significantly changing the predetermined shape of the pulses to obtain an amplified pulse train. Each of the amplified pulses has a substantially square temporal power density distribution, a sharp rise time, a pulse duration and a fall time. The method also includes delivering and focusing at least a portion of the amplified pulse train into a spot on the target material wherein the rise time is fast enough to efficiently couple laser energy to the target material, the pulse duration is sufficient to process the target material and the fall time is rapid enough to prevent the undesirable changes to the material surrounding the target material.
The target material may include microstructures such as conductive lines or links, the latter being common circuit elements of redundant semiconductor memories. The conductive lines may be metal lines and wherein the pulse duration is sufficient to effectively heat and vaporize the metal lines, or a specified portion thereof.
The target material may be a part of a semiconductor device such as a semiconductor memory having 16-256 megabits.
At least a portion of the material surrounding the target material may be a substrate such as a semiconductor substrate.
The target material may be part of a microelectronic device.
The substantially square temporal power density distribution is sufficient to substantially completely ablate the target material.
Preferably, the rise time is less than 1 nanosecond and, even more preferably, is less than 0.5 nanoseconds.
Preferably, the pulse duration is less than 10 nanoseconds and, even more preferably, is less than 5 nanoseconds.
Also, preferably, the fall time is less than 2 nanoseconds.
A single amplified pulse is typically sufficient to process the target material.
The target material may have a reflectivity to the amplified pulses and wherein the power density of the amplified pulses is sufficiently high to reduce the reflectivity of the target material to the amplified pulses and to provide efficient coupling of the laser energy to the target material.
Preferably, each amplified pulse has a relatively uniform power density distribution throughout the pulse duration.
Preferably, each pulse has a temporal power density distribution uniform to within ten percent during the pulse duration.
The material surrounding the target material may have optical properties, including absorption and polarization sensitivity, and thermal diffusivity properties different from the corresponding properties of the target material.
Preferably, the repetition rate is at least 1000 pulses/second and each of the amplified pulses has at least 0.1 and up to 3 microjoules of energy.
Preferably, the step of optically amplifying provides a gain of at least 20 DB.
Also, preferably, both the rise time and the fall time are less than one-half of the pulse duration and wherein peak power of each amplified pulse is substantially constant between the rise and fall times.
Preferably, each of the amplified pulses has a tail and the method also includes attenuating laser energy in the tails of the amplified pulses to reduce fall time of the amplified pulses while substantially maintaining the amount of power of the pulses.
Still further in carrying out the above objects and other objects of the present invention, an energy-efficient system for processing target material having a specified dimension in a microscopic region without causing undesirable changes in electrical or physical characteristics of material surrounding the target material is provided. The system includes a controller for generating a processing control signal and a signal generator for generating a modulated drive waveform based on the processing control signal. The waveform has a sub-nanosecond rise time. The system also includes a gain-switched, pulsed seed laser having a wavelength for generating a laser pulse train at a repetition rate. The drive waveform pumps the laser so that each pulse of the pulse train has a predetermined shape. Further, the system includes a laser amplifier for optically amplifying the pulse train to obtain an amplified pulse train without significantly changing the predetermined shape of the pulses. Each of the amplified pulses has a substantially square temporal power density distribution, a sharp rise time, a pulse duration and a fall time. The system further includes a beam delivery and focusing subsystem for delivering and focusing at least a portion of the amplified pulse train onto the target material. The rise time is fast enough to efficiently couple laser energy to the target material, the pulse duration is sufficient to process the target material, and the fall time is rapid enough to prevent the undesirable changes to the material surrounding the target material.
The laser amplifier preferably includes an optical fiber and a pump such as a laser diode to pump the optical fiber wherein the pump is distinct from the seed laser.
The laser diode pump source may also be gain switched (pulsed and directly modulated) to increase diode lifetime by switching to the xe2x80x9coffxe2x80x9d state during extended periods where laser processing is not occurring.
Preferably, the seed laser includes a laser diode.
The system may include an attenuator for attenuating laser energy in the tails of the amplified pulses to reduce fall time of the amplified pulses while substantially maintaining the amount of energy of the pulses.
The pulse duration may be chosen as a function of a specified target material dimension. The specified material dimension may be less than the laser wavelength. The preferred laser is a high speed, semiconductor laser having a wavelength less than about 2 xcexcm. Future material advances in semiconductor laser diode technology and fiber materials may provide for operation in the visible region as well as at longer infrared wavelengths.
The seed laser diode may be a multimode diode laser or a single frequency (single mode) laser utilizing a distributed Bragg reflector (DBR), distributed feedback (DFB), or an external cavity design.
The spot size typically has a dimension in the range of about 1 xcexcm-4 xcexcm.
The density of the memory may be at least 16-256 megabits.
The semiconductor device may be a microelectromechanical device.
Preferably, the attenuated laser energy in the pulse tail is attenuated by at least 10 dB within 1.5 times the pulse duration.
Yet still further in carrying out the above objects and other objects of the present invention, an energy-efficient, laser-based method for ablating a metal link having a specified dimension embedded in at least one passivation layer without causing undesirable changes in electrical or physical characteristics of the at least one passivation layer surrounding the metal link is provided. The method includes generating a laser pulse train utilizing a laser having a wavelength at a repetition rate. Each of the pulses of the pulse train has a predetermined shape. The method also includes optically amplifying the pulse train without significantly changing the predetermined shape of the pulses to obtain an amplified pulse train. Each of the amplified pulses has a substantially square temporal power density distribution, a sharp rise time, a pulse duration and a fall time. The method further includes delivering and focusing at least a portion of the amplified pulse train into a spot on the metal link. The rise time is fast enough to efficiently couple laser energy to the metal link. The pulse duration is sufficient to ablate the metal link and the fall time is rapid enough to prevent the undesirable changes to the at least one passivation layer surrounding the metal link.
Still further in carrying out the above objects and other objects of the present invention, an energy-efficient system for ablating a metal link having a specified dimension embedded in at least one passivation layer without causing undesirable changes in electrical or physical characteristics of the at least one passivation layer surrounding the metal link is provided. The system includes a controller for generating a processing control signal and a signal generator for generating a modulated drive waveform based on the processing control signal. The waveform has a sub-nanosecond rise time. The system also includes a gain-switched, pulsed seed laser having a wavelength for generating a laser pulse train at a repetition rate. The drive waveform pumps the laser so that each pulse of the pulse train has a predetermined shape. Further, the system includes a laser amplifier for optically amplifying the pulse train without significantly changing the predetermined shape of the pulses to obtain an amplified pulse train. Each of the amplified pulses has a substantially square temporal power density distribution, a sharp rise time, a pulse duration and a fall time. The system further includes a beam delivery and focusing subsystem for delivering and focusing at least a portion of the amplified pulse train into a spot on the metal link. The rise time is fast enough to efficiently couple laser energy to the metal link. The pulse duration is sufficient to ablate the metal link, and the fall time is rapid enough to prevent the undesirable changes to the at least one passivation layer surrounding the metal link.
The metal link may be embedded in a top passivation layer thereover and a bottom passivation layer thereunder. The pulse duration is sufficient to crack the top passivation layer but not the bottom passivation layer.
In carrying out the above objects and other objects of the invention a method is provided to ablate target material using a laser having a wavelength suitable for laser material processing while avoiding damage to surrounding materials. The method includes the steps of modulating a laser beam to produce a predetermined gain-switched pulse and focusing the laser beam onto the target region. The predetermined gain-switched pulse shape includes a rise time of the laser pulse fast enough to efficiently couple laser energy to a target structure, with a pulse duration of sufficient length to efficiently heat and vaporize the target material, and a pulse decay time which is rapid enough to avoid damage of structures surrounding the target material.
Further in carrying out the above objects and other objects of the invention a system for is provided to ablate material using a laser having a wavelength suitable for laser processing while avoiding damage to surrounding materials. The system includes a laser source, components to modulate the laser source to generate a laser pulse having a predetermined gain-switched pulse shape, and optical components for focusing the laser beam onto the target region. The predetermined pulse shape includes an optical rise time of the laser pulse fast enough to efficiently couple laser energy to a target structure, with a pulse duration of sufficient length to efficiently heat and vaporize the target material, and a pulse decay time which is rapid enough to avoid damage of structures surrounding the target material.
In a preferred construction of the invention, the gain-switched pulse shape includes a fast rise time pulse, substantially flat at the top, with a fast pulse fall time. A xe2x80x9cseedxe2x80x9d laser diode is directly modulated to generate a predetermined pulse shape. The optical power is increased through amplification with a fiber laser amplifier to output power levels sufficient for laser processing. The resulting gain-switched pulse at the fiber laser amplifier output is focused onto the target region.
In a construction of the invention, it can be advantageous to directly modulate the xe2x80x9cseedxe2x80x9d diode to produce a predetermined gain-switched square pulse and provide low distortion amplification using a fiber laser amplifier to provide output pulse levels sufficient for material processing.
In an alternative construction, the pulse temporal power distribution of the directly modulated seed diode is modified to compensate for distortion or non-uniformity of the fiber amplifier or other components, for instance the xe2x80x9csmoothxe2x80x9d rise of an output modulator. The resulting laser processing pulse which is focused into the target region will have a desired shape: fast rise time, relatively flat during the pulse duration, with rapid decay.
In a construction of the invention it can be advantageous to enhance the performance of the laser processing system by providing a xe2x80x9cpulse slicingxe2x80x9d module which is used to attenuate laser energy remaining at the output of the laser processing system when the xe2x80x9cseedxe2x80x9d laser pulse is terminated, thereby preventing heating of sensitive structures not designated as target material after processing is complete. The xe2x80x9cpulse slicingxe2x80x9d technique is useful to attenuate the tail of either a modified pulse or a standard Q-switched pulse. This is illustrated in FIGS. 4a and 4b, wherein a log scale is provided in the vertical axis of FIG. 4b. 
It is preferred to perform laser processing operations, particularly metal link blowing, at pulse rates of at least 1 KHz (1000 pulses/second) with laser pulse energy of at least 0.1 microjoules in a pulse, the 0.1 microjoules being emitted at the output of the fiber amplifier, where the fiber optic amplifier gain is at least 20 DB (1000:1).
In a construction of the invention, a laser pulse is shaped having a rise and fall time shorter than about one-half of the pulse duration and where the peak power is approximately constant between the rise and fall time.
In a construction of the invention, it is possible to generate a series of closely-spaced, short pulses which, when combined, produce a desired pulse shape as illustrated in FIGS. 3a and 3b. 
In a construction of a system using the invention it can also be advantageous to operate the laser at pulse repetition rates exceeding the material processing rate and utilize a computer controlled optical switch to select processing pulses, the computer being operatively connected to a beam positioning system used to position a focused laser beam for material processing.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.